The present invention relates to a safety PLC having duplex power supply circuits.
In recent years, PLCs (safety PLCs) that have duplex hardware or software configurations to improve reliability have come into widespread use. An example of the safety PLC is disclosed in the following Patent Document 1.
FIG. 5 shows an example of the hardware configuration of a safety PLC according to the related art. FIG. 5 shows systems X and Y and a signal transfer path provided between the systems X and Y. The systems X and Y realize a duplex system in a single safety PLC. The systems X and Y are supplied with power from DC power supplies of different systems. Therefore, for example, in a case in which a short circuit occurs in a power supply circuit for supplying power to the system Y and thus a DC voltage of 24V is applied to the signal transfer path, if the resistance value of a resistor Ro included in the signal transfer path is set to 12 kΩ or more, the current flowing through a transistor Tra included in the system X can be kept smaller than or equal to 2 mA.
In the safety PLC according to the related art, generally, a wiring line provided with a resistor having a resistance value of, for example, about 10 KΩ is used in the signal transfer path between the above-mentioned systems X and Y. In other words, according to the related art, when a wiring line provided with the resistor Ro with a high resistance value is used in the signal transfer path connecting the systems, even if a power supply circuit of one of two systems (for example, the system Y) fails, a circuit (for example, the transistor Tra) of the other system can be reliably protected.
Patent Document 1: JP-A-2002-358106
However, when a wiring line provided with a resistor having a high resistance value of about 10 KΩor more is used in the signal transfer path connecting the systems X and Y, stray capacitance that is parasitic on the high-resistance circuit makes it difficult to raise the frequency of a digital signal (square wave) to be transmitted through the signal transfer path above that in the related art. This phenomenon occurs because the stray capacitance that is parasitic on the high-resistance circuit, such as an MELF resistor, serves as a low pass filter and thus the waveform of the digital signal (square wave) to be transmitted is distorted.